1. Field of the Invention
The present invention relates to a lens system installed in a mobile communication terminal, and more particularly to a lens system with a high performance and compact structure, which comprises a small number of lenses and is installed in a mobile communication terminal.
2. Description of the Related Art
Recently, mobile communication terminal products, in which a camera is integrally installed in a mobile communication terminal, have become popular with consumers. In such a camera, a subject is photographed using a lens attached to an image pickup device such as a CCD or a CMOS, and data of the photographed subject are recorded on a recording medium.
Further, a digital still camera, in which a still image formed by the lens is electrically accepted by the image pickup device including the CCD (hereinafter, referred to as “CCD”) and then recorded by an internal memory or memory card, uses a liquid crystal monitor as a finder when photographing or as a reproducing monitor for reproducing the photographed image. Accordingly, compared with a silver halide camera, the digital still camera has advantages such as immediateness and convenience, while having a disadvantage of a lower resolution of the photographed image.
A lens system installed in a mobile communication terminal or a digital camera for photographing a subject must satisfy requirements, as follows.
Since an image pickup device has a pixel size of less than 4 μm so as to allow the camera to have a mega pixel capacity, the lens system must be designed such that it has a sufficient resolution and a size larger than the actual size of a sensor in consideration of an assembly tolerance.
Further, the lens system must have TV distortion of less than 1%. The more corrected the distortion is, the greater the deterioration in the resolution becomes. Accordingly, preferably, the TV distortion is less than 1%.
Moreover, since the amount of a marginal ray of the mobile communication terminal is affected by the angle of light being incident on the sensor and the amount of a marginal ray of the lens system, the lens system must be designed at a designated angle such that light can be incident on the sensor, thereby allowing the amount of a marginal ray of the camera to be increased.
In case that the lens system is designed at an angle smaller than the desired angle, the resolution, distortion and the length of the optical system in the lens system are deteriorated, thereby causing a problem of combining the lens system with the sensor.
In the lens system installed in the mobile communication terminal or digital camera for photographing a subject, there occur various aberrations, such as spherical aberration, astigmatism and distortion, in which the shape of an input image formed by photographing the subject is deformed by the influence of the incident light having one type of wavelength. Accordingly, the lens system must be designed such that it suppresses the occurrence of the various aberrations.
Here, chromatic aberration is a failure of a lens or spherical mirror to focus particles of different energies to the same point on an axis based on the wavelength of light when the lens or spherical mirror forms an image of a subject. Spherical aberration generally means other aberrations except for the above chromatic aberration, and strictly means aberration due to the failure of a lens to focus particles at different lateral distances from the axis thereof to the same point longitudinally on the axis, i.e., for a converging lens and particles incident upon the lens parallel to the axis, particles farther from the axis are focused nearer the lens than particles closer to the axis.
Further, distortion is a failure of a lens, in which a straight line of a subject is bent to form an image. In the formed image, the straight line is bent inwards like a spool or outwards like a wine barrel. The distortion occurs when the magnification of the subject is in proportion to the distance, while the formed image is not in proportion to the distance.
Moreover, astigmatism, widely speaking, is one of the spherical aberrations, in which an image on an object space separated from an axis is not converged into a complete point, but diverged in a ring-like or radial shape.
Korean Patent Application No. 2002-0005279 discloses a lens system designed such that it satisfies the above-described requirements. As shown in FIG. 1, the lens system disclosed by this application comprises a perforated iris STO arranged closest to a subject, a first lens group LG1, a second lens group LG2 and a third lens group LG3, which are sequentially arranged from the subject. The first lens group LG1 includes a first lens L1 having a positive refractivity and a second lens L2 having a negative refractivity attached or separated from the first lens. The second lens group LG2 includes a third lens L3 having a positive refractivity and including at least one aspherical refractive plane. The third lens group LG3 includes a fourth lens L4 having a negative refractivity and including at least one aspherical refractive plane. Further, this application discloses the prior art of a conventional camera lens.
That is, the lens system in accordance with Korean Patent Application No. 2002-0005279 comprises the first lens group LG1 having a high positive power, the second lens group LG2 having a positive power, and the third lens group LG3 having a low negative power, thus obtaining a long-focus type optical power arrangement in order of the positive power, the positive power and the negative power. Further, in order to correct chromatic aberration, achromatism is achieved by the first lens group LG1 having the comparatively high power. Accordingly, the first lens group LG1 serves to mainly correct spherical aberration, coma-aberration and chromatic aberration generated around an optical axis, and the second and third lens groups LG2 and LG3 serve to correct distortion, i.e., non-axial aberration, and to maintain good tele-centricity.
As described above, the conventional lens system employs a designated number of lenses, that is, four lenses, thereby increasing the volume of an optical system, and cost and time taken to perform a complicated assembly process.
Further, the perforated iris STO of the conventional lens system is arranged closest to the subject, thereby having a limit in shortening the length of the optical system employing this lens system. Accordingly, using the above-described conventional lens system, it is difficult to manufacture a small-sized mobile communication terminal having a size of less than predetermined dimensions.